This invention relates to a method for three dimensional image reconstruction from data acquired in a positron emission tomograph ("PET"), typically for medical imaging purposes.
In positron emission tomography ("PET"), the location of a positron source within the tissues of a living subject is determined by detection of the oppositely directed gamma rays, known as "lines of response" or "coincidence lines", which result from so-called "annihilation events", electron collisions with emitted positrons. By simultaneously detecting the location of the two gamma rays, information may be derived concerning the location of the positron source. The aggregate of such information can be used to construct an image.
Energy carried by the gamma rays is typically sensed by detectors disposed in an array about the subject under study. The detectors convert the energy carried by the gamma rays, to record the position of the events which gave rise to the rays. Electrical signals representative of the detected gamma rays may be processed by a system which typically includes a programmed digital computer capable of processing the position data to form an image of the structure, organ or patient under examination.
Since the gamma radiation resulting from positron annihilation travels approximately collinearly and in opposite directions, sensing coincidence in time of rays extending in opposite directions establishes the occurrence of an event, and, as will be described, the location of the event as well.
Numerous positron emission tomography scanners have been described in the prior art.
Most commercially available PET scanners have used detectors comprised of many discrete scintillation crystals, optically coupled to individual photomultiplier tubes disposed around the field of view containing the image volume. Some prior art attempts to enhance image resolution in PET have involved the use of larger numbers of detectors, or alternatively, a reduced although still numerous number of detectors mounted in a movable array.
G. Muehllehner et al. have heretofore proposed and constructed a positron tomograph capable of achieving high spatial resolution while avoiding the complexity of individual photomultipliers ("PMTs") coupled to individual small detectors. G. Muehllehner, J. S. Karp, D. A. Mankoff et al.: Design and Performance of a New Positron Tomograph. IEEE Trans. Nucl. Sci., Vol. 35, No. 1, pp. 670-674 (1988). Such devices feature and are capable of exploiting large area modular position-sensitive detector crystals, coupled to an array of PMTs. In one such type of device, produced and sold by UGM Medical Systems, Inc., of Philadelphia, Pa., as the "PENN-PET" scanner, a hexagonally arranged array of six large area position-sensitive sodium iodide (NaI(T1)) crystals is used, each crystal being coupled to an array of thirty PMTs arranged in three rows of ten. J. S. Karp., G. Muehllehner, D. A. Mankoff et al.: Continuous Slice PENN-PET: A Positron Tomograph With Volume Imaging Capability. J. Nucl. Med., Vol. 31, No. 5, pp. 616-627 (1990).
Detectors of the above type have good spatial resolution in both the transverse and the axial directions, and the axial field of view of the detectors has been increasing steadily. Such detectors can identify a very large number of different coincidence lines through the three-dimensional space between opposing detector modules.
In devices such as the above-mentioned PENN-PET scanner, sensed coincidence lines are not constrained, as by septa or other means, to planes perpendicular to the longitudinal (z) axis of the field of view. Indeed, in such scanners the coincidence lines are oblique for most detected photon pairs, and reconstruction algorithms must therefore be devised to handle the fully-three dimensional ("3D") raw data from the detector system.
Two different approaches have been used to reconstruct from raw data. These may be called for convenience the "single-slice rebinning" approach and the "fully-3D" approach. The single-slice rebinning approach assigns each oblique coincidence line to a particular two dimensional ("2D") sinogram associated with a transverse slice of the volume. The reconstruction of the resulting stack of sinograms proceeds in a slice-by-slice fashion using conventional 2D algorithms known to those skilled in the art. This approach involves certain geometrical approximations (See: M. E. Daube-Witherspoon and G. Muehllehner, Treatment of Axial Data In Three-Dimensional PET, J. Nucl. Med., Vol. 28, pp. 1717-1724 (1987)), but it requires the least possible amount of computation and data storage capability. Fully-3D reconstruction algorithms, both iterative and non-iterative, process oblique coincidence lines without making geometrical approximations, but image reconstruction using either kind of fully-3D algorithm requires a considerable amount of computation.